The present invention relates to an EL (electroluminescence) display device, more particularly, to a thin-film EL display device having a high-contrast ratio adapted to preventing contrast from being reduced by either a external light or a light generated from a fluorescent layer itself.
Conventionally, the ELD (electroluminescence display) generally having been used as a plane display device has been known as an alternative of CRT (cathode ray tube) in terms of highly increased luminance, longer life and lower drive voltage in spite of technical difficulties, because it is a self-luminescent device and an all-solid state device, differently from the existing LCD (liquid crystal display).
A typical example of the above-mentioned EL display device will now be described with reference to FIG. 1.
As shown in FIG. 1, the EL display device comprises a transparent substrate 1. A transparent electrode 2 of a uniform width is formed by first depositing a transparent layer of ITO (Indium Tin Oxide) over the upper surface of the transparent substrate 1 to have a surface resistance of 10.OMEGA. per unit area by using a chemical vapor depositing method or a sputtering method and then line-etching said ITO layer by using a photoetching method. Next, a first insulating layer 3 of a uniform thickness is formed by depositing a dielectric material such as Y.sub.2 O.sub.3, SiO.sub.2, and Si.sub.3 N.sub.4 over the upper surface of the transparent electrode 2, by using an EB (electron beam) method or the sputtering method. Thereafter, a fluorescent layer 4 is formed by depositing ZnS:Mn over the upper surface of the first insulating layer 3 by using the EB method or the sputtering method. On the upper surface of the fluorescent layer 4, a second insulating layer 5 is formed in the same manner as that of the first insulating layer. Finally, a background electrode 6 is formed by depositing an Al layer over the upper surface of the second insulating layer 5 by using the EB method or the sputtering method and thereafter line-etching said Al layer in vertical direction with respect to the transparent electrode 2. In the drawing, reference numeral 7 designates a power supply.
In the above-mentioned conventional EL display device, when a high voltage of AC 200V is applied between the transparent electrode 2 and the background electrode 6, electrons are trapped in the first and second insulating layers 3 and 5. On the other hand, electrons existing at the interfaces between the insulating layers 3 and the fluorescent layer 4 and between the insulating layer 5 and the fluorescent layer 4 are activated, by a high intensity of induced electric field, to be injected into the fluorescent layer 4. Upon entering the fluorescent layer 4, these electrons impact the luminescent centers of Mn atoms in ZnS of the fluorescent layer 4, thereby causing outer electrons in the Mn atoms to be excited. Then, the excited electrons in the Mn atoms are returned to their ground states releasing whatever energy they have, resulting in emission of light with a wavelength corresponding to an energy level gap. If the fluorescent materials are composed of ZnS; Mn, an intrinsic wavelength of 5850 .ANG. is typically generated from Mn atoms.
The light emitted in the above-mentioned manner passes through the transparent electrode 2 toward the viewer. The light arrived at the background electrode 6 is reflected on the surface of the background electrode 6 to at least 90% to be again directed to the viewer.
Incident external light entering the transparent substrate 1 passes through each thin film layer formed on the transparent substrate 1 and is reflected at the background electrode 6 to be directed forwardly. However, this reflected light interferes with light emitted from the fluorescent layer 4, because of an extremely high transparency of its each thin film layer, thereby causing the contrast ratio between lighting pixel and unlighting pixel to be considerably lowered. Especially when ambient light is very bright, the displaying ability of the device is very reduced since luminance of the EL light and the reflected light are approximately equal to each other.